Inrush current is generally a momentary input current surge in an electrical device occurring when the power control device is first activated. The term is also sometimes used to refer to the maximum, instantaneous input current drawn by an electrical device when first turned on. Although inrush current typically reduces to a lower steady-state device current (e.g., during normal device operation), the effects of inrush current can be significant. Inrush currents can damage device components, lower supply voltage available to other circuits, decrease power efficiency, cause system errors, make devices difficult to control, and complicate device design. For example, if the input power source for a device is current limited or has poor load regulation, large inrush currents can cause the input voltage to drop significantly, thereby affecting overall device operation. While many devices are subject to problematic inrush currents, DC/DC converters and power amplifiers are particularly susceptible because of input capacitor and/or filter element charging that occurs when power is first provided to such devices.
Soft-start circuits can be used to alleviate the problems associated with inrush currents. This is typically accomplished by ramping up the output of the power control device (e.g., a DC/DC converter) at a rate slower than would otherwise occur without the soft-start circuit. Numerous different types of soft-start circuits have been implemented including, for example: soft-start circuits that delay full current output of a power converter by linearly increasing a pulse width modulator (PWM) pulse width; soft start circuits that ramp a reference voltage provided to an error amplifier from zero to its nominal value, thereby easing the output voltage up at a slower rate; and soft-start circuits that limit the device loop until the soft start voltage is higher than the desired voltage reference (usually from bandgap reference) to the error amplifier, while the desired output is reached.
When applied to existing integrated circuit devices such as DC/DC converter ICs, traditional soft-start circuits utilize one or more off-chip components such as capacitors, and typically require the use of one or more additional IC package pins for controlling the inrush current. Accordingly, it is desirable to have integrated circuit devices that include soft-start circuitry, thereby reducing system cost, simplifying system design and reducing the number of needed components. Moreover, it is further desirable to improve the quality of soft-start circuit operation.